Once the gp120 drops down and binds with CD4 and the co-receptor, the HIV spike is exposed. The spike is a glycoprotein with a molecular weight of 41 kilodaltons.

This gp41 molecule acts much like a spring-loaded lancet. Once triggered into action by the attachment of the gp120 to the CD4 and co-receptor, the gp41 protein springs out and pierces the membrane of the host cell.